1. Field of Invention
The invention relates to a dual-band voltage controlled oscillator and, in particular, to a dual-band voltage controlled oscillator utilizing the switched feedback loop technology.
2. Related Art
Due to rapid development in wireless communication systems, there is an increasing need for integrating multiple bands in communication system chips. Therefore, it is important in radio-frequency transceiving circuits to provide a dual-band oscillating source or an oscillator circuit with wide-band oscillating sources.
Generally speaking, there are two kinds of Colpitts voltage controlled oscillators in a dual-band circuit: one is the capacitor switched Colpitts voltage controlled oscillator, and the other is the inductor switched Colpitts voltage controlled oscillator.
FIG. 1 shows a capacitor switched Colpitts voltage controlled oscillator 100. It includes a first switch device 120, a second switch device 130, an inductor 170, a capacitor 160, a current source 190, a first N-type transistor 102, a second N-type transistor 103, a third N-type transistor 104, two capacitors 140, 150, and a variable capacitor 180.
When we change the capacitances of the capacitors 140, 150 in the resonance cavity, the oscillating frequency of the oscillator also changes accordingly. As we switch between the switch devices 120, 130 in the switching circuit 110, we obtain different equivalent capacitances, thereby obtaining different oscillating frequency bands. This enables the oscillator to work in the desired bands.
However, such a circuit design has a big flaw. That is, it is impossible to obtain optimized inductance and capacitance in the bands. For example, if the inductance and capacitance in the resonance circuit are designed for the resonance frequency in a higher band, the equivalent capacitance has to be increased in order to obtain a lower frequency. Therefore, a lot of parasite capacitors with fixed values have to used, resulting in the decrease in the tunable high-frequency band. This also increases the power consumption and lowers the quality factor Q (Q factor) of the resonance cavity at the same time.
FIG. 2 depicts an inductor switched Colpitts voltage controlled oscillator 200 with mainly the equivalent inductance inside the resonance cavity thereof. The inductor switched Colpitts voltage controlled oscillator 200 includes a switch device 220, another switch device 230, a first N-type transistor 201, a second N-type transistor 202, a third N-type transistor 203, two inductors 250, 240, a capacitor 260, a variable capacitor 280, and a current source 290. The basic design idea of this oscillator is to change the equivalent inductance using the switch devices. By switching between the switch device 220 and the switch device 230 of the switching circuit 210, different equivalent inductances can be obtained, thereby rendering different oscillating bands. This enables the oscillator to work in the desired bands.
However, the drawback of this circuit is that the inductor in the resonance cavity is the passive device with the worst Q factor. By connecting a switch device in series to the inductor will result in a parasite resistance in the switch device, affecting the overall Q factor. This seriously decreases the Q factor of the resonance cavity. The extra resistance and capacitance also increase the power consumption.
It is thus highly desirable to provide a wide-band voltage controlled oscillator that can solve the above-mentioned problems.